Nanocomposites are a relatively new class of composites that are particle-filled polymers for which at least one dimension of the dispersed particle is in the nanometer range (10−9 meter). Because of the size of the dispersed particles, certain nanocomposites can exhibit improved mechanical, thermal and optical properties as compared to pure polymers or conventional composites.
The most commonly used and investigated types of polymer nanocomposites are those based on clays and layered silicates. The nanocomposites are typically obtained by the intercalation or penetration of the polymer (or a monomer subsequently polymerized) inside galleries of layered clay material and the subsequent exfoliation, or dispersion, of the intercalate throughout the polymer matrix.
The matrix polymer of the polyolefin resin blend typically constitutes a nonpolar, organic material, whereas the clay mineral is a much more polar, inorganic material. This incompatibility hinders the direct intercalation or exfoliation of the clay in the typical polymer blend. Various methods have been tried in the prior art in an attempt to overcome this problem and thereby increase the dispersion of the clay mineral in the polymer matrix.
For example, U.S. Pat. No. 5,910,523 discloses a composition comprising a semi-crystalline polyolefin, a clay filler having dispersible platelets in stacks, an amino-functional silane reacted with the filler, and a carboxylated or maleated semi-crystalline polyolefin that has been reacted with the amino-functional silane after the silane was reacted with the filler.
U.S. Pat. No. 6,228,903 discloses a composition made by contacting a phyllosilicate material that is exfoliated in an organic solvent with a polymer/carrier composition that includes a water-insoluble polymer and a solvent, whereupon the adherent solvent is driven off.
U.S. Pat. No. 6,451,897 discloses a composite material made in a substantially non-oxidizing environment by graft polymerizing a liquid monomer onto a propylene resin in the presence of smectite clay and a free radical initiator. The propylene resin is a porous material, wherein more than 40% of the pores have a diameter greater than 1 micron. The liquid monomer may be a vinyl-substituted aromatic, a vinyl ester, or an unsaturated aliphatic nitrite or carboxylic acid.
U.S. Pat. No. 6,462,122 discloses a nanocomposite blend containing a layered silicate material, a matrix polyolefin, and a functionalized polyolefin (i.e., maleic-anhydride-modified polypropylene) that may be blended together in, for example, a twin-screw extruder.
Despite these prior art formulations, however, incorporating clay minerals into a polymer matrix may not always result in markedly enhanced mechanical or physical properties. This may be due in part to the inability to exfoliate all or at least a substantial portion of the layers of the silicate material. It may also be due in part to the lack of affinity between the layered silicate materials and the organic polymers. There remains a need to obtain polymeric materials that have a good level of mar/scratch resistance along with the physical property requirements of rigidity, strength, processability, barrier properties, clarity, gloss, and thermal stability.